The present invention relates to circuits for protecting systems which include diode switching devices against the effects of diode reverse recovery currents which occur upon the switching of a diode.
One of the serious problems involved in electrical circuits including switching devices and diodes, such as power supplies, inverters, motor drivers, and the like, is the problem of reverse recovery current resulting when the diode is switched off, which causes the current through the diode to be momentarily reversed. Thus, when a diode that was forwardly biassed is switched from forward conduction to reverse the diode behaves as a short circuit during the first few nanoseconds or microseconds until all the stored charge in the diode is recovered. This causes an extremely high magnitude of current, called reverse recovery current, to flow through the diode and the switching device during this recovery period. This recovery current places a considerable stress on the diode and the switching device, and also wastes a considerable quantity of energy in the form of heat. This recovery current also causes energy accumulated in effective inductors and capacitor of the circuit to be radiated as electrical noise.
One known solution for this problem of diode recovery is to include an inductor in series with the electrical switch. While this solution protects the electrical switch, it does not solve the problem of wastage of energy. In addition, the inductor generates high-voltage spikes, and therefore it is common to include a zener diode to absorb the excess voltage, but this also does not solve the problem of wastage of energy. The common solution is to use an ultrafast-acting diode, but such diodes are very expensive, if available at all, for particular applications producing a relatively high forward voltage drop.
The probelm is even more severe when using MOSFET's (metal oxide semi-conductor field effect transistor), particularly in bridge configurations. Thus, if the integral Drain/Source is allowed to conduct, the whole device is limited upon reapplying the dv/dt. In order to stay at a safe level of dv/dt, it is necessary to add a large serial inductance and large parallel capacitors. These components cause an additional waste of power. Another solution is to prevent conduction in the diode in the MOSFET by inserting a serial diode, to block backward conduction, and bypassing the MOSFET diode combination by an additional diode, parallel to the combination. However, these solutions are expensive and significantly increase the circuit losses.